The first thing everyone knows about Richard Wagner is that he was anti-Semitic. The Israeli Knesset throws a fit any time a visiting conductor strikes up the Tristan prelude, and apologists like the philosopher Brian Magee tie themselves in tiny knots trying to prove that Wagner did not really want to eliminate Jews ... just to squeeze out giants like Heine and Mendelssohn from his "greater" scheme of things.

Richard Wagner's anti-Semitic work was one of the pre-cursors of German Nazism

The evidence of race-hate is abundant. Wagner, in 1850, wrote an anonymous essay, Das Judentum in der Musik, arguing that Jews were ineligible to practise art. He later had the tract reissued under his own name. He abused Jews routinely, according to the diaries of his second wife, Cosima, and based the devious characters of Mime and Beckmesser, in part at least, on caricatures of his least favourite Jews.

The Nordic myths of Wagner's operas helped form Adolf Hitler's outlook. Magee, and Thomas Mann before him, argued that Wagner was Hitler's first victim, his music hijacked by an evil ideology. But no study of Nazism can avoid Wagner's influence. He provided, unwittingly, the cultural legitimisation for genocide.

Wagnerian vileness pervaded a second generation. Wagner's son Siegfried, with his English wife Winifred, were among Hitler's earliest supporters. The couple invited him to Bayreuth in October 1923 and, after the following month's failed Munich putsch, supplied him in jail with home comforts and, it is said, with the paper on which he wrote Mein Kampf. Bayreuth during the Third Reich was a national shrine. The family continues to this day to suppress documentation.

The evidence they conceal, however-may be less incriminating than they think. Take Siegfried, the son and heir. He would have been a disappointment to his gritty old dad, who died when he was just 13. Siegfried wrote soft centred romantic operas, had little interest in world dominance and fathered a love child with the local pastor's wife; the boy was later employed at Bayreuth as a stagehand. This was a youthful aberration. The adult Siegfried was mostly gay, actively so. Bayreuth's treasurer paid off a string of would-be blackmailers.

Fired by Hitler's fervid personality, Siegfried designated the 1924 Ring cycle "a German redemption festival" and festooned it with nationalist tokens. But the more his wife swooned over the F¸hrer, the cooler Siegfried grew. After his death in 1930, the Nazis took possession of his papers from Winifred (contrary to rumour, she never seduced Hitler; her lover was the collaborationist stage director, Heinz Tietjen).

The Siegfried papers went to Berlin, where they were captured by the Russians in 1945. They were found in Moscow at the end of last year by a scholar from the Washington Holocaust Museum, who copied 100,000 pages and put some of the best bits online. Siegfried's private observations amount to an unqualified renunciation of Wagnerian racism, a point he made most trenchantly in heated exchanges with a certain Dr Salomon, the rabbi of Bayreuth.

The rabbi had written first to Siegfried in June 1924, complaining of a rise in "anti-Semitic influences" at the Wagner house. Siegfried replied: "We are against the Marxist spirit, [but] we have nothing against patriotic Jews ... Although I have found regrettable things in my father's handwriting, I have no bad feelings for Jews - that must be understood. Some of them have been of great help to me in my work."

Siegfried maintained that his father's rhetoric had been hotheaded and untrue to himself. For all his condemnation of " Jewishness" in art, Wagner had specifically chosen a trinity of Jews to present Parsifal, his quasi-religious opera. Josef Rubinstein had prepared the piano score, Heinrich Porges led the choir and Hermann Levi, a rabbi's son, conducted the premiere. Wagner, argued Siegfried, was no racist in the Hitler sense of the word.

The rabbi wrote back, demanding to know exactly where Siegfried stood on Nazism. "Art reaches us through the heart," replied Siegfried equivocally, "and God gave hearts to all human beings." A year later he was more forthright, assuring the rabbi that, contrary to newspaper reports, he had not dropped a Jewish singer, Friedrich Schnorr, because Hitler was in attendance. No Jew would be excluded from Bayreuth. "Anyone who wants can come to the festival, whether Hitler or [the pacifist] Harden," he wrote. In a further letter, he sent the rabbi a pair of tickets.

Siegfried appears to have been a benign nationalist of the old school, more German unifier than racial purifier. He claimed his father had been the same. His view of Wagner was anathema to the Nazis, who confiscated his archive, and to his widow, who suppressed his works; his opera, Bruder Lustig, is about to appear on record for the first time.

So if the Wagners, man and boy, were basically decent chaps, where did the xenophobia spring from? The finger of suspicion points to the bitter widows, neither of whom was German. Cosima, half-French daughter of the Hungarian Franz Liszt, dominated Bayreuth until her death in 1930.

Winifred, born in Hastings, died an unrepentant Nazi in 1980. Wagner's image was spun by these women for a century after his death. The finding of Siegfried's letters is the first chink in these old wives' tales, the first chance to put their version of history into perspective - and possibly into the dustbin.